Summary Reactive oxygen species (ROS), endothelial injury and macrophages play critical roles in ischemia/reperfusion (I/R) kidney injury. Our data show stanniocalcin-1 (STC1) diminishes superoxide generation in macrophages, through induction of uncoupling protein-2 (UCP2), decreases the response of macrophages to chemoattractants- and migration across an endothelial monolayer. In cultured endothelial cells, STC1 preserve barrier function. STC1: diminishes superoxide generation; inhibits cytokine-induced activation of Jun-N- terminal kinase (JNK) and loss of tight junction proteins expression. STC1 transgenic mice, which exhibit elevated serum levels and preferential expression of STC1 in macrophages and endothelium, display resistance to I/R kidney injury. Overall hypothesis: STC1 protects from I/R kidney injury through: suppression of superoxide generation; maintenance of normal endothelial barrier function following I/R kidney injury; and inhibition of macrophages. In Objective I, we will determine the role of superoxide and Daxx in STC1-mediated inhibition of JNK in endothelial cells. In Objective II, we will determine the effect of STC1 on hypoxia/reoxygenation (H/RO)-induced changes in the expression and assembly of tight junction proteins in cultured primary kidney endothelial cells. In the context of I/R kidney injury, Objective III will examine endothelial leakage to macromolecules, kidney inflammation and function after kidney endothelium-specific or macrophage-specific overexpression or deletion of STC1. Few therapeutic options are currently available for acute kidney injury (AKI). Our data identify STC1 as a potential therapeutic target for ischemic injury in the kidney and other organs, and our proposed studies will further elucidate STC1 mechanisms of action.